PROBABILITY AND ENTROPY 147 



If we take a still smaller weight the chance 

 goes up very rapidly, and if we examine par- 

 ticles which are just visible with a good micro- 

 scope we find them hopping about in what is 

 known as the Brownian movement, and which 

 we now know to be identical with that thermal 

 agitation which all molecules or groups of mole- 

 cules exhibit, and which always increases with 

 increasing temperature. From a study of these 

 movements Perrin was able to draw some ex- 

 tremely important deductions with the aid of 

 the same formula as I have just employed. 



If the jumping of the one-gram weight was 

 a miracle, here in the Brownian movement thou- 

 sands of such miracles are constantly occurring 

 before our eyes. It is not true that things left 

 to themselves approach a constant state, but 

 only that they approach a state which ordi- 

 narily appears constant to us because of the 

 dullness of our perceptions. 



It was Maxwell who made the ingenious sug- 

 gestion of a little demon who could see and dis- 

 tinguish between the individual molecules. Let 

 us consider, as a type of irreversible phenome- 

 non, the mixing of two gases. A container with 

 a partition in the middle has oxygen on one 

 side, nitrogen on the other. If a small hole is 



